This invention relates to switching systems in general and more particular to switching systems which employ groups of different priority subscribers. As is well known in military switching systems and other secured type of switching systems, there exists groups of different priority subscribers. As such, these subscribers have different security levels and different priorities. A common example of such a switching system is widely employed by the military and, for examples, use red phone subscribers, which may be analog subscribers, as well as black phone subscribers, which may be both analog and digital subscribers. As such certain red phone subscribers, for example, may be higher priority subscribers than other red phone subscribers. In this manner certain telephone subsets associated with one priority subscriber are connected to various modems or encryption equipment whereby the telephone transmissions are decoded or converted to various forms which include scrambling or other encoding or encryption techniques to render secure communications thereby assuring that other unauthorized subscribers receive unintelligible data or that conventional eavesdropping techniques cannot be employed. As one can ascertain, certain conversations which may emanate over such switching systems are extremely confidential and hence should be guarded against by eavesdropping in regard to unauthorized users. In any event, as indicated, most of these switching systems utilize subscriber groups with different security levels. These subscriber groups are able to contact one another through a master switching matrix or a switching network. In this manner it is possible for STET to occur due to the common connections through the switching matrix. One practice of the prior art is to class mark subscribers in distinct groups and to prevent one group from calling another. In any event, this is not sufficient as cross talk and false connections due to normal operation of the switching matrix can compromise a critical call if no protection against it is provided.
The physical separation of terminal equipment tends to minimize the danger of cross talk, however, as both user groups must use the common switching network the danger of false routing still exists. Such switching networks utilize PCM or pulse code modulated switching systems and hence offer the opportunity to use the systems encoding/decoding circuits to process the signals of the different user groups. Selective processing makes it more difficult for one user group to understand the signals of the other. In any event, it does not in any manner effect the signal transmission through the switching network.
Hence the prior art proposed a partitioning of a secured switch into black and red groups by using separate class marks for the different groups. This allows the restriction of call set ups to users of the same class only. In any event, all switching systems will provide certain numbers of false connections. Hence, the protection against false connections or the transfer of classified information to unauthorized users is a critical problem. Thus, in order to avoid the communications of such critical information, the prior art envisioned many different proposals. According to one aspect, the users of the red group employ in their PCM codecs a reversed algorithm. Essentially, this implies a special PCM codec chip that is required for all red subscribers and each of the red line circuits must be equipped with a special codec chip. In this manner it requires that such common elements, such as switching system operators, must have two separate connections which is one for the red subscriber and one for the black subscriber. While the technique allows the unimpaired use of the switching network because only the voice data element of the transmission packet is changed, it requires different codecs or chips in each of the lines. This concept allows the transposition of data in the bit stream to prevent an authorized user from reconstructing and understanding a misrouted message. Similarly, a digital user with a data terminal will also receive unintelligible information if, in fact, he is improperly connected to an unauthorized line. In any event, it is understood that non-sequential connections, such as only connections between red subscribers and such as only connections between black subscribers, are relatively expensive to implement and to guard against.
It is therefore an object of the present invention to provide a simplified approach to prevent misdirected messages from being understood by different priority subscribers in a switching system.
It is a further object of this invention to provide a simple and inexpensive approach to prevent unauthorized subscribers from receiving messages that can be reconstructed and which may contain critical data.